docs: add dual-domain implementation plan

docs/superpowers/plans/2026-05-16-dual-domain.md

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+# Dual-Domain Support Implementation Plan
+
+> **For agentic workers:** REQUIRED SUB-SKILL: Use superpowers:subagent-driven-development (recommended) or superpowers:executing-plans to implement this plan task-by-task. Steps use checkbox (`- [ ]`) syntax for tracking.
+
+**Goal:** Serve the portfolio on both `gabrielkaszewski.dev` and `gabrielkaszewski.pl` with identical content, with `.dev` as the canonical domain for SEO.
+
+**Architecture:** Two independent changes — Traefik routing config to accept both hostnames, and Next.js metadata to inject a canonical `<link>` tag. No app logic changes.
+
+**Tech Stack:** Next.js 15 (App Router metadata API), Traefik (Docker labels), Let's Encrypt TLS.
+
+---
+
+### Task 1: Add `.pl` domain to Traefik router
+
+**Files:**
+- Modify: `compose.yml`
+
+- [ ] **Step 1: Update the Traefik router rule**
+
+In `compose.yml`, change the router rule label from:
+
+```yaml
+- "traefik.http.routers.gabrielkaszewski.rule=Host(`gabrielkaszewski.dev`)"
+```
+
+to:
+
+```yaml
+- "traefik.http.routers.gabrielkaszewski.rule=Host(`gabrielkaszewski.dev`) || Host(`gabrielkaszewski.pl`)"
+```
+
+The full `labels` block should look like:
+
+```yaml
+labels:
+  - "traefik.enable=true"
+  - "traefik.http.routers.gabrielkaszewski.rule=Host(`gabrielkaszewski.dev`) || Host(`gabrielkaszewski.pl`)"
+  - "traefik.http.routers.gabrielkaszewski.entrypoints=websecure"
+  - "traefik.http.routers.gabrielkaszewski.tls.certresolver=letsencrypt"
+```
+
+Note: Traefik's Let's Encrypt cert resolver automatically provisions a cert for every hostname matched by the router rule, so no additional TLS labels are needed.
+
+- [ ] **Step 2: Commit**
+
+```bash
+git add compose.yml
+git commit -m "feat: add gabrielkaszewski.pl to traefik router"
+```
+
+---
+
+### Task 2: Add canonical URL to Next.js metadata
+
+**Files:**
+- Modify: `app/layout.tsx`
+
+- [ ] **Step 1: Add `alternates.canonical` to the metadata export**
+
+In `app/layout.tsx`, add `alternates` to the existing `metadata` object:
+
+```ts
+export const metadata: Metadata = {
+  title: {
+    default: "Gabriel Kaszewski | Software Engineer",
+    template: "%s | Gabriel Kaszewski",
+  },
+  description:
+    "The portfolio of Gabriel Kaszewski, a software engineer specializing in Rust, Python, and modern web technologies.",
+  keywords: [
+    "Gabriel Kaszewski",
+    "Software Engineer",
+    "Rust Developer",
+    "Python Developer",
+    "Next.js",
+    "Portfolio",
+  ],
+  alternates: {
+    canonical: "https://gabrielkaszewski.dev",
+  },
+  openGraph: {
+    // ... rest unchanged
+  },
+  // ... rest unchanged
+};
+```
+
+- [ ] **Step 2: Verify the canonical tag is present in the built HTML**
+
+Run the dev server:
+
+```bash
+bun run dev
+```
+
+Then in another terminal, check the rendered HTML for the canonical tag:
+
+```bash
+curl -s http://localhost:3000 | grep -i canonical
+```
+
+Expected output:
+
+```
+<link rel="canonical" href="https://gabrielkaszewski.dev"/>
+```
+
+- [ ] **Step 3: Commit**
+
+```bash
+git add app/layout.tsx
+git commit -m "feat: add canonical URL to metadata"
+```